diff --git a/packages/core/agent/src/inbox.ts b/packages/core/agent/src/inbox.ts
index 2ffc4bed39..5487a843c9 100644
--- a/packages/core/agent/src/inbox.ts
+++ b/packages/core/agent/src/inbox.ts
@@ -128,9 +128,13 @@ export class Inbox {
 
   /** Read the current durable projection state. */
   private current(): InboxState {
-    // AgentLoop requires sessionProjections; AgentRegistry contributes this unit to it.
-    // oxlint-disable-next-line typescript/no-non-null-assertion
-    return this.ctx.sessionProjections.stateOf(this.session, 'inbox')!
+    const state = this.ctx.sessionProjections.stateOf(this.session, 'inbox')
+    if (state === undefined) {
+      throw new Error(
+        `agent "${this.session.id}" cannot read inbox state: session projection "inbox" is not registered; load AgentRegistry with SessionProjectionRegistry before constructing Inbox`,
+      )
+    }
+    return state
   }
 
   /** Commit one normalized mutation and publish its live events. */
diff --git a/packages/core/agent/tests/agent.spec.ts b/packages/core/agent/tests/agent.spec.ts
index f6a82a7a7d..a6e8e68659 100644
--- a/packages/core/agent/tests/agent.spec.ts
+++ b/packages/core/agent/tests/agent.spec.ts
@@ -74,6 +74,17 @@ async function reconstructPersistedInbox(
 }
 
 describe('Inbox', () => {
+  it('reports a missing Inbox projection as a composition error', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SessionProjectionRegistry)
+    const agent = stubAgent('missing-inbox-projection', { ctx })
+    const inbox = new Inbox(ctx, agent.session, agentEvents(ctx, agent))
+
+    expect(() => inbox.nextTurn).toThrow(
+      'agent "missing-inbox-projection" cannot read inbox state: session projection "inbox" is not registered; load AgentRegistry with SessionProjectionRegistry before constructing Inbox',
+    )
+  })
+
   it('rejects invalid durable coordinates and duplicate identities during reconstruction', async () => {
     const outOfRange = await reconstructPersistedInbox('invalid-inbox-range', (session) => {
       session.append('agent/inbox/spliced', {